This invention relates generally to magnetic field producing means and is concerned more particularly with means for radially aligning powdered particles of annular magnetic devices.
Generally, in powder metallurgical fabrication of magnetic devices, such as by sintering, for example, suitable consitituent materials are mixed to produce a solid composite material, which then is comminuted to a fine powder. The powder particles may be compacted into a desired annular configuration by conventional means, while subjecting the particles to a radially directed magnetic field. After compacting, the packing density of the powder material may be about 75% of the theoretical maximum value, as determined by dividing the weight per unit volume by the density of the material. Subsequently, when the compacted device is heated to a suitable sintering temperature, the powder material undergoes shrinkage and further densification, whereby the packing density may increase to as much as 97% of the theoretical maximum value. As a result, the aligned powder particles are bonded to one another; and the magnetic properties of the composite material are greatly enhanced. Accordingly, when the annular device is magnetized in the direction of the particle alignment, a device having high magnetic coercivity and energy product is produced.
Thus, in order to produce a strong radially polarized magnet, it is necessary to align the maximum number of powder particles in the radial direction during the compacting operation. However, during compacting, it may be found that the intensity and shape of the particle aligning magnetic field, particularly above twenty kilogauss, for example, is severely limited by magnetic core saturation. As a result, a significant number of the powder particles may remain axially oriented; and the required high densification may not be achieved.
Therefore, it is advantageous and desirable to provide improved magnetic field aligning means for orienting powder particles in the radial direction during processing of an annular magnetic device.